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How to Use the Pressure Enthalpy Calculator
Select the refrigerant in the system. For blends the calculator uses the dew point for superheat and the bubble point for subcooling.
Enter the suction pressure in PSIG and the superheat measured at the evaporator outlet. Use the superheat calculator if you only have a line temperature.
Enter the discharge pressure in PSIG, the discharge line temperature in °F and the liquid line subcooling. Enter the system capacity too if you want mass flow, heat rejected and compressor power.
The results show the four enthalpies, refrigeration effect, heat of compression, heat of rejection, cycle COP, isentropic efficiency, compression ratio and the flash gas share at the evaporator inlet.
Reading the refrigeration cycle on a P-H diagram
The pressure-enthalpy diagram maps every state point in the refrigeration cycle. The horizontal axis is enthalpy, the energy each pound of refrigerant carries, and the vertical axis is absolute pressure. The dome in the middle is the saturation envelope: liquid sits to its left, superheated vapour to its right, and a liquid and vapour mixture inside it.
The four state points
Point 1 is the evaporator outlet, superheated vapour at low pressure. Point 2 is the compressor discharge, superheated vapour at high pressure and high temperature. Point 3 is the condenser outlet, subcooled liquid at high pressure. Point 4 is the metering device outlet, a low-pressure mixture of liquid and vapour that has the same enthalpy as point 3 because expansion adds or removes no heat. In the R-410A example, with 118 PSIG suction, 12°F superheat, 400 PSIG discharge, 180°F discharge temperature and 12°F subcooling, the enthalpies are 123.8, 143.6, 53.5 and 53.5 BTU/lb.
Enthalpy units and the reference state
Enthalpy has no absolute zero, so every chart fixes a reference. This calculator sets the enthalpy of saturated liquid at -40°F to zero, and only differences between points matter for capacity and COP, so a chart with a different reference gives the same refrigeration effect. Charts in SI units show kJ/kg instead of BTU/lb, and the enthalpy converter moves values between them.
Refrigeration effect: what the evaporator delivers
Refrigeration effect is the enthalpy difference between point 1 and point 4, which is 70.2 BTU/lb in the example. A larger value means each pound of refrigerant carries more heat out of the space, so the compressor moves less mass for the same capacity. The 27.9% flash gas share at point 4 is the part of each pound that evaporates during expansion without cooling anything, and more subcooling shrinks it. Superheat counts toward the refrigeration effect only when it is picked up in the evaporator, so superheat gained in an uninsulated suction line adds no useful cooling.
Heat of compression and isentropic efficiency
Heat of compression is the enthalpy rise from point 1 to point 2, 19.8 BTU/lb in the example. The ideal rise, if compression were isentropic, is smaller, and the ratio of the two is the isentropic efficiency, 70.5% here. A high compression ratio from low suction pressure or high discharge pressure raises the heat of compression and lowers the cycle COP. The heat of rejection calculator turns the heat of rejection into a condenser load.
COP from the cycle
Cycle COP is the refrigeration effect divided by the heat of compression, 3.55 for the example against an ideal value of 5.03. Entering a capacity of 36,000 BTU/hr gives a mass flow of 512.6 lb/hr, 46,152 BTU/hr of heat rejected and 2.98 kW of compressor power into the refrigerant. To compare with an EER rating, the EER calculator converts between EER and COP.
Why the cycle COP is not a seasonal rating
This COP describes one operating point at the pressures you measured, so it will not match a seasonal figure such as the SEER2 ratings that Natural Resources Canada publishes for central air conditioners. A seasonal rating averages many outdoor temperatures and includes fan power and cycling losses. Use the cycle COP to compare a system with itself before and after a repair.
Blends, dew points and bubble points
For a blend the saturation temperature depends on whether you are at the dew point or the bubble point. The calculator reads the evaporating dew point from the suction pressure and adds the superheat to it, and it reads the condensing bubble point from the discharge pressure and subtracts the subcooling from it. Discharge superheat is measured from the condensing dew point. Take the superheat and subcooling from the superheat calculator and subcooling calculator using the same convention.
Property data and accuracy
Enthalpies come from property tables generated with CoolProp, an open-source fluid property library, and checked against its full equation of state. The tables agree to within about 0.5 BTU/lb on refrigeration effect, and far closer on heat of compression. The NIST REFPROP database is the reference many engineers use for refrigerants and mixtures, and it is the better choice for design work that needs a stated uncertainty. Gauge pressures are converted with 14.696 psi of atmospheric pressure, so readings taken at altitude carry a small error.
Frequently Asked Questions
A pressure-enthalpy (P-H) diagram, also called a Mollier diagram, plots absolute pressure on the vertical axis against enthalpy on the horizontal axis. The refrigeration cycle is a loop on it: evaporation at low pressure, compression to high pressure, condensation, and expansion back down. The width of the evaporator line is the refrigeration effect, and the width of the compression line is the heat of compression. Technicians use the diagram to find capacity, COP and mass flow, and to see which part of the cycle has changed when a system performs badly.
Refrigeration effect is the enthalpy leaving the evaporator minus the enthalpy of the liquid entering the metering device. Convert each gauge pressure to absolute pressure, find the saturation temperatures, add the measured superheat to the suction dew point and subtract the measured subcooling from the condensing bubble point, then read the enthalpies at those states. For R-410A at 118 PSIG suction with 12°F superheat and 400 PSIG discharge with 12°F subcooling, the refrigeration effect is 70.2 BTU/lb. Multiply by the mass flow in lb/hr to get the capacity in BTU/hr.
Use the manufacturer's data for the pressures you are running, because it changes with the compression ratio and the compressor type. This calculator finds the apparent value from the discharge temperature: it divides the ideal compression work by the actual heat of compression. In the R-410A example that is 70.5%. A discharge probe that loses heat to the air reads low and makes the efficiency look better than it is, so insulate the probe and place it close to the compressor.
Blends such as R-407C and R-448A do not boil or condense at one temperature, so the temperature at a given pressure depends on whether the refrigerant is all vapour or all liquid. Superheat is measured from the dew point and subcooling from the bubble point. At 250 PSIG discharge the condensing dew point of R-407C is 115.8°F and the bubble point is 107.2°F, a gap of 8.7°F, while R-410A differs by only 0.2°F. The calculator uses the right one for each measurement.